The present invention relates to a micromechanical sensor element having a diaphragm which is fixed in a frame.
Such a sensor element may be configured as microphone, for example. Microphones are usually made up of two capacitor plates, one of the plates being realized as a thin diaphragm which is displaced relative to the second plate by the sound pressure. In the capacitor the deformation of the diaphragm is detected in the form of the resulting charge transfer. In the case of a condenser the charges are supplied by an external voltage source, in the case of an electret the charges are fixedly implanted in a plastic foil.
In the micromechanical microphones known from practice the deflection or deformation of the diaphragm is usually detected in a capacitive and not a piezoresistive manner since the conventional piezoresistive pressure sensors do not exhibit sufficient sensitivity in the pressure range of only 10 Pa and below which is of relevance here. One possibility for increasing the sensitivity of the known pressure sensors is to design a thinner and larger diaphragm. However, when reducing the thickness of the diaphragm it must be taken into account that the piezoresistors have a depth extension of approx. 3μ in most cases. That is to say, the diaphragm should have a thickness of at least 12μ since the piezoresistors would otherwise come to lie in the vicinity of the neutral axis. The diaphragm surface also can be increased only to a limited extent since non-linear effects could otherwise occur in the measured-value acquisition and, furthermore, not only the size of the sensor element would increase but the cost of the entire sensor system as well.